The invention relates generally to cardiac pacers, and more particularly to means for preventing crosstalk between bipolar pacer leads.
There are two major pumping chambers in the heart, the left and right ventricles. Simultaneously contracting, these chambers expel blood into the aorta and the pulmonary artery. Blood enters the ventricles from the left and right atria, respectively. The atria contract in a separate action which precedes the major ventricular contraction by an interval of about 100 milliseconds (ms), known as the AV delay. The contractions arise from a wave of electrical excitation which begins in the right atrium and spreads to the left atrium. The excitation then enters the atrio-ventricular (AV) node which delays its passage via the bundle of His into the ventricles. Atrial contractions begin every 400-1,000 ms at a metabolically determined frequency known as the "sinus" rate.
Electrical signals corresponding to the contractions appear in the electrocardiagram. A signal known as the P-wave accompanies atrial contraction while a signal known as the QRS complex, with a predominant R-wave, accompanies the ventricular contraction.
The typical implanted cardiac pacer operates by supplying missing stimulation pulses to provide excitation via an insulated wire (or "pacing lead") terminating in an electrode attached to the right ventricle. The naturally occurring R-wave can be reliably detected by the same lead to inhibit or trigger stimulation or to restart a timing interval as in "demand" pacing. An additional pacer lead contacts the atrium to sense P-waves, if desired. Pacers whose ventricular stimulation is timed from the sensing of a P-wave are referred to as AV synchronous or "physiological" pacers since they preserve the natural sinus rate as well as the normal sequence of contractions. In AV sequential pacers, the atrial lead is used for atrial stimulation. Examples of physiological AV sequential pacers or "double demand" pacers in which the atrial and ventricular leads can both stimulate and sense are shown in pending U.S. patent application Ser. No. 153,422 entitled "Ventricular Inhibited Cardiac Pacer" filed May 27, 1980 and U.S. patent application Ser. No. 207,003 entitled "Multi-Mode Microprocessor Based Programmable Cardiac Pacer" filed Nov. 14, 1980, both assigned to the assignee of the present application, and incorporated herein by reference in their entirety.
There are two types of electrode systems used in pacing leads. Unipolar leads terminate distally in a single electrode (cathode) and employ the case of the pulse generator itself, or a conductive plate on the case, as the return electrode or ground (anode). Bipolar pacing leads, on the other hand, teminate distally in two spaced insulated electrodes connected to the pulse generator through respective wires in the pacing lead. Thus, each bipolar lead carries a positive and negative electrode for the respective chamber, and the case is not designed to form a part of the electrical circuit in this configuration. The two positive electrodes on the respective bipolar leads are tied together electrically through a common ground connection.
In an AV sequential bipolar lead pacing system, this shared ground connection can present crosstalk problems in both sensing and stimulation when each bipolar lead is in a different heart chamber. This is an extremely important problem to solve for physiological pacers which provide bipolar stimulation and sensing for both heart chambers wth the same pacer powered by a single battery.
One of the ways previously used to accomplish some measure of isolation between bipolar leads is to employ a transformer in the output stage of the pacing circuit to isolate the lead electrodes. This approach, however, has the serious drawback of allowing pacing isolation, but not allowing sensing isolation. In addition, it necessitates adding a relatively bulky, inefficient and noise susceptable component to the otherwise miniaturized pacer electronics.